I agree with what 1fish2fish said. Many people make the mistake of not doing this, and then their plants die due to lack of wattage. When you figure out the wattage, it will open you up to a good idea of what you can get.
You also need to be sure you get the correct light bulb for your tank. Some tanks come with incandescent light bulbs. You ned to get daylight spectrum CFL bulbs for live plants. Also, like Dramaqueen said, it does make your cycling process easier.